A laminate film 1 to be used for forming a resist film for plating or etching, when a printed wiring board is prepared, is constituted by coating a photosensitive resin composition on a support (base) layer (a base film) 2', followed by drying, to form a photosensitive layer 3, and laminating a coating (covering) layer (a protective film) 4' on the photosensitive layer 3 as shown in FIG. 2(a). In the prior art, the support layer 2' and the photosensitive layer 3 form a transfer layer 5', and the transfer layer 5' is laminated on a substrate 6 of a printed wiring board, on which the transfer layer 5' is to be laminated.
When the conventional laminate film 1 is to be laminated, after peeling (stripping) the coating layer 4', the transfer layer 5' is superposed on the substrate 6 so that the photosensitive layer 3 faces toward the side of the substrate 6, and then the transfer layer 5' is pressurized from the side of the support layer 2' by using a heating roll to effect contact bonding. Therefore, a sectional view after lamination is as shown in FIG. 2(b).
Next, a negative mask (phototool) is placed on the support layer 2', and the photosensitive layer 3 is exposed by irradiation with light for exposure through the negative mask Thereafter, the negative mask is removed, the support layer 2' is peeled (stripped) off, and then the photosensitive layer 3 is developed to obtain the photosensitive layer 3 having the same pattern as that of the above negative mask. By using the photosensitive layer 3 left on the substrate as a resist film, a next plating or etching step is carried out.
As the support layer 2', a film (e.g., a film of polyethylene terephthalate (PET)) having a 5% elongation load in the longitudinal direction of the film per unit width at 80.degree. C. of 100 g/mm or more is used, and the thickness of the film is generally about 20 .mu.m. In order to heighten the tensile strength of the laminate film 1, the support layer 2' is required to have such a thickness, and it is also necessary to increase the hardness of the support layer 2' to a certain extent.
The photosensitive layer 3 is formed from a photosensitive resin composition in which the physical properties of an irradiated portion are changed when UV ray or the like is irradiated, and a suitable composition is selected depending on the purpose of use. The thickness of the photosensitive layer 3 is set to be, for example, 25 .mu.m, 33 .mu.m, 40 .mu.m or 50 .mu.m depending on the purpose. As the coating layer 4', a film of polyethylene or the like is used, and the thickness of the film is, for example, 30 .mu.m.
When the transfer layer 5' is laminated, it should be conformed to the unevenness of a base material so that a non-adhered portion between the photosensitive layer 3 and the base material does not exist.
In recent years, the density of wiring (line and space) of a printed wiring board has been heightened, and high resolution has been demanded. In order to heighten the resolution of the laminate film 1, it is effective to make the photosensitive layer 3 thinner. However, the amount of the photosensitive layer which is conformed to the surface unevenness of a base material is reduced so that in the conventional laminate film 1, a non-adhered portion between the substrate 6 and the transfer layer 5 is enlarged to cause a problem that sufficient production yield cannot be obtained. Also, in the conventional laminate film, the flexibility of the entire transfer layer 5' is made insufficient due to the above thickness and hardness necessary for the support layer 2', and the transfer layer 5' is difficultly conformed to the surface unevenness of a base material on which the transfer layer 5' is to be laminated. As a result, a non-adhered portion between the substrate 6 and the transfer layer 5' is enlarged to cause a problem that sufficient production yield cannot be obtained.
Various means to cope with the problems as described above have been proposed. For example, a method of coating water on a base material and then laminating a laminate film on the base material has been described (see Japanese Provisional Patent Publications No. 21890/1982 and No. 21891/1982).
In this method, in order to adhere a thin water layer to the base material uniformly, the surface of the base material should be cleaned. When through holes with a small size or the like exist in the base material, water held in the through holes and a photosensitive layer are easily reacted to involve drawbacks that developability is lowered and so on.
Also, a method of laminating a laminate film on a base material after forming an adhesive intermediate layer by laminating a liquid resin on the base material has been proposed (see Japanese Provisional Patent Publication No. 154363/1977).
In this method, there are drawbacks that the developability, peeling property (stripping property) and so on of through holes with a small size are lowered, cost is increased by coating the liquid resin, and so on.
Also, a method of laminating a laminate film under reduced pressure by using a vacuum laminator has been known (see Japanese Patent Publication No. 31670/1978 and Japanese Provisional Patent Publication No. 63702/1976).
In this method, an apparatus is expensive, and evacuation takes time. Therefore, the method is not used frequently for forming a common circuit and is utilized only as a laminate of a permanent mask to be used after formation of a conductor. Also, in such lamination of a permanent mask (solder mask), further improvement of conformability to a conductor has been demanded.
An object of the first invention is to provide a laminate film in which a transfer layer is favorably conformed to the surface unevenness of a target on which the transfer layer is to be laminated, resolution can be improved and the production yield of a printed wiring board can be greatly improved without increasing production costs.
An object of the second invention is to provide a laminate film having improved sensitivity, crosslinking density, storage stability and resolution and good peeling (stripping) property of a first film from a photosensitive layer in addition to the effects of the first invention.
An object of the third invention is to provide a laminate film having excellent handling property in addition to the effects of the first and the second invention.
An object of the fourth invention is to provide a laminate film having improved operatability at the time of lamination in addition to the effects of the third invention.
An object of the fifth and the sixth inventions are to provide processes for preparing a printed wiring board, in which a transfer layer is favorably conformed to the surface unevenness of a target on which the transfer layer is to be laminated, resolution can be improved and the production yield of a printed wiring board can be greatly improved without increasing production costs.